1. Field of the Invention
The present invention relates generally to a hydraulically operated braking system for motor vehicles, and more particularly to improvements in such a braking system of a type which is capable of performing both an anti-lock or anti-skid braking pressure control, and a traction control of drive wheels of the vehicle.
2. Discussion of the Prior Art
The anti-lock braking pressure controlled is effected to prevent the vehicle wheels from skidding on a road surface upon locking of the wheels due to an excessive braking torque applied to the wheels, in relation to the friction coefficient of the road surface. On the other hand, the traction control of the drive wheels is effected to avoid a phenomenon that the vehicle cannot be suitably or effectively accelerated due to slipping of the drive wheels on the road surface, which is caused by an excessive driving torque applied to the drive wheels, in relation to the friction coefficient of the road surface. A hydraulically operated anti-lock braking system having a traction control function is disclosed in laid-open publication No. 57-22948 (published in 1982) of an unexamined Japanese Patent Application.
In the anti-lock braking system disclosed in the above-identified publication, a pilot-operated directional control valve is provided so that a reservoir and a traction control hydraulic power source are selectively brought into communication with a discharge port of a solenoid-operated pressure regulating device. A brake fluid pressure in a master cylinder of the braking system is employed as a pilot pressure applied to the pilot-operated directional control valve. While the fluid pressure in the master cylinder is relatively high, the directional control valve is placed in a position for fluid communication between the discharge port of the pressure regulating device and the reservoir, so that an anti-skid braking pressure control operation may be performed. While there exists no pressure build-up in the master cylinder with the brake pedal placed in the non-operated position, the directional control valve is placed in a position for fluid communication between the discharge port of the pressure regulating device and the traction control hydraulic power source, so that a traction control of the drive wheels may be achieved. During the traction control operation, the brake fluid is discharged from wheel cylinders (which apply brake to the drive wheels) and is returned to the master cylinder through a fluid passage which connects the pressure regulating device and the master cylinder. In the disclosed braking system, a pump of an anti-lock control hydraulic power source and a pump of the traction control hydraulic power source are incorporated in a modular unit, and are driven by a single common drive motor.
In the braking system described above, the anti-lock control device and the traction control device are elaborately assembled into a relatively compact assembly which is capable of performing both an anti-lock braking pressure regulating operation and a traction control operation. Accordingly, the cost of manufacture of the braking system as a whole can be reduced if all types of a given model of a motor vehicle are equipped with the anti-lock control device and the traction control device.
However, there is a requirement that some types of the same model of vehicle are equipped with only the anti-lock control device while the other types are equipped with both the anti-lock control device and the traction control device. For the vehicles equipped with only the anti-lock control device, the above-indicated assembly of the two devices cannot be employed, and a separate arrangement capable of performing only an anti-lock control operation should be prepared. Therefore, the overall cost of manufacture of the braking systems for the same model of vehicle is raised.